This invention relates to heat-curable epoxide resin compositions, to a process for their cure, and to cured products obtained by this process.
It is known that epoxide resins, i.e., substances containing on average more than one 1,2-epoxide group per molecule, may be cured by reaction with various classes of substances to form cross-linked, infusible, insoluble products having valuable technical properties.
U.S. Pat. No. 3,386,956 discloses the curing of epoxide resins by means of ureas represented by the following formulae ##STR2## wherein
R represents a divalent radical selected from the group consisting of methyl-substituted phenylene, methylenediphenylene, dimethoxydiphenylene, and dimethyldiphenylene,
R.sup.1 represents methyl or hydroxyethyl,
n is an integer of from 2 to 5, and
X represents --OCH.sub.3, --Cl, --H, --CH.sub.3, or --NO.sub.2.
The United States patent also discloses that the curing effect of these agents may be promoted by incorporating dicyandiamide, stearic hydrazide, adipic dihydrazide, succinimide, or cyanoacetamide in the curable compositions.
According to that patent, use of the aforesaid ureas overcomes a difficulty associated with curable epoxide resin compositions of the prior art, namely, that relatively high temperatures and relatively long curing times were needed if curing agents such as dicyandiamide were used, whereas if other, more reactive, curing agents or accelerators were used they caused premature curing at room temperature, i.e., they were not sufficiently latent.
U.S. Pat. No. 3,660,316 discloses that 1-cyano-3-(lower alkyl)-guanidines also promote the curing of epoxide resins by bisureas of formula II to IV.
Ureas of the foregoing formulae, in particular N-(4-chlorophenyl)-N',N'-dimethylurea and 2,4-bis(N,N-dimethylureido)toluene, have been used commercially to cure epoxide resins, alone or with dicyandiamide as promoter, but also as promoters of the curing of epoxide resins with dicyandiamide.
However, because of increasingly severe requirements for curable compositions having long storage lives at room temperature but curing rapidly at moderately elevated temperatures (e.g., 100.degree. C.), replacements for the above-mentioned ureas are now being sought.
We have now found that certain phenolic ureas substantially meet these more stringent requirements.